Shadowfell

Also known as The Underdark, The Twilight Realm, Realm of the Raven Queen, The Vale of Shadows, and by certain religions: purgatory. It is a realm that exists as a shadow of Nevalia, where rot and decay have corrupted every fiber of the world. It is said to be the place where souls go once they've died, before Kelemvor, God of the Dead determines their fate.

For a brief time, Myrkul, Lord of the Dead, had been granted the power to pull souls back into Nevalia from Shadowfell. This led to the creation of undead creatures. Before Myrkul fell to Bhaal, Lord of Death it is said that he brought back millions of haunted souls to our world. Some can even be found today in the form of geists, ghosts, skeletons, zombies, and many others.

Since then, many magic-wielders have found various other ways to interact with Shadowfell. This practice is known as necromancy.

It is believed that Myrkul was able to use his power over the plane to escape Shadowfell long after his death. This event would be known as the beginning of the plot of the Mad Bard.

In the centuries since Bhaal, God of Murder had killed Kelemvor, it has been unclear who has been ruling Shadowfell and who is now determining which souls may pass on. The only lead comes from small cults who seem to be worshiping a deity known as the Raven Queen. Within these groups, Shadowfell is even referred to as The Realm of the Raven Queen.

It's known that during the Age of Undoing, a group known as The Fell began to appear within Shadowfell. They are thralls to someone or something known as the Flayer of the Fell.